The near-universal symbol of drunk driving enforcement is a motorist standing on the side of the road blowing into a handheld breath measurement device. Yet at the same time, you have likely also heard that breath tests are typically not reliable as evidence when it comes to supporting drunk driving charges. How is it, then, that a law enforcement officer can use such a measurement to justify your arrest for drunk driving? 

A more apt question may be why is your breath considered to be a good indicator of what the alcohol content of your blood is? When you ingest ethanol alcohol (the type of alcohol used in drinks), the compound is able to enter into the bloodstream by passing through the walls of the stomach and intestines through a process known as passive diffusion. According to the Alcohol Pharmacology Education Partnership, it is then carried throughout the body in the blood via the capillaries. Passive diffusion continues to occur throughout this journey, with some traces of ethanol eventually ending up in the thin mucous layer of the alveolar sacs of your lungs. These contain the oxygen that you inhale. This oxygen disrupts the hydrogen bonds within ethanol causing it to vaporize into a gaseous form. That vaporized ethanol then leaves the body as you exhale carbon dioxide. 

This process continues with every breath, with the ethanol concentration in your blood slowly lowering as you breathe. The fluid nature of this process contributes to the unreliability of breath testing, as it is next to impossible to determine a consistent rate at which blood-alcohol content lowers through this process. Knowing this may offer added support to you challenging drunk driving charges if they are solely based on the results of breath testing.